Polyester base

Polyethylene terephthalate polyester has better strength and dimensional
stability than cellulose triacetate. Today it is not used much for
motion picture film [except for Super 8 where its strength helps to
reduce the risk of film damage] Polyester base cannot be cement-spliced
like other film bases and tape splicing and heat splicing are used to
join film.
Polyester film is known by a number of trade names including Kodak
"Estar" and 3M's Mylar, and is also often referred to as "Ester" base.
This is by far the most stable and resistant substance used for film
base, it seems substantially inert, lower in flammability than cellulose
acetates and it is dimensionally very stable and less sensitive to
changes over time. Polyester film base does have a number of drawbacks,
the most important being that it is far stronger than any other base and
this can lead to damage to equipment as the film doesn't break in fault
conditions. Equipment for handling polyester base has to designed with
this in mind and a good example is a Super 8 projector that has only a
single sprocketed drive roller. Old equipment designed for acetate bases
can be seriously damaged by polyester.
It is not possible to break polyester film by hand and scissors must be
used. Once nicked at the edge, polyester film base can be torn by hand,
a little unwillingly, and leaves a sharp jagged cutting edge.
Polyester film also burns although it normally melts first and requires
furnace temperatures to destroy entirely.